When I use to carry a P7M8 I used a Rosen ARG-Plus, but not the slimline version. I loved the holster and found it completely comfortable and very concealable. It was so comfortable that I never bothered getting an OWB holster for that gun. I liked the offset loop and felt like the holster was able to shift as needed. And putting the extra leather at the bottom was a great idea and essential to the comfort.
In fact, having that holster made me stop using Summer Special type IWB's, and I've tried to use the ARG styles whenever possible. It's only in the last few months, since joining this and other forums, that I've started looking at dual loop IWB's, but that's another story.
I don't know how much thinner the slimline could be or if it's worth the extra cost, but I'm sure that you'll be very happy with the ARG-Plus.